Influence of Chemical Ordering on Field-Induced Strain in Ni-Mn-Ga Alloys

技术：铁磁形状记忆合金（fsma）已被广泛研究，特别是自1996年在-150℃下观察到Ni2MnGa晶体中0.2%的磁场诱导应变以来。最近，在非化学计量学中，在室温下观察到6%和9.6%的马氏体成分，分别具有四方5-M结构和正交7-M结构。这些大的场致应变的机制被认为是在一定的磁场方向下，由磁能差和双边界上的转矩驱动的双边界运动。虽然现存的现象学理论解释了应变和磁化的一般场依赖性，但驱动孪晶边界运动的微观机制尚未得到很好的理解。也不清楚是否，如果是，如何，化学计量成分中的化学顺序和非化学计量成分中的化学顺序影响控制双边界运动的部分位错的成核或动力学。这项拨款支持多方面的实验和建模方法来解决这些问题。中子散射在NIST（或洛斯阿拉莫斯）进行，以确定典型5-M和7-M成分的统计位点紊乱。这些成分中总是含有过量的Mn和缺乏的Ga， Ni一般在48%到51%之间。这决定了多余的Mn原子倾向于驻留在Ni或Ga位点上。将尝试确定在孪晶边界积聚的物质，这些物质可能解释在这些材料中观察到的磁场或应力诱发蠕变。用差示磁强计研究了过量Mn占据Ni或Ga位点对饱和磁化强度和磁各向异性的影响。在这种金属间Heusler成分中，Mn原子携带了大部分的磁矩。因此，可以预计，无论Mn原子占据Ni还是Ga位点，它们都将更靠近其他Mn原子，因此可能存在磁矩下降。部分无序组合物中位置对称性的降低也可能导致磁各向异性的降低，这对场致孪晶界运动至关重要。在Ni-Mn-Ga晶体上施加小的声信号，诱导平行于孪晶面的剪切运动，对磁场诱导的孪晶面边界运动有显著的增强作用。这些变化是用变化化学计量学来研究的。实验结果解释了在演变的微观模型，部分位错介导的双边界运动。非技术：这项研究有望对Ni-Mn-Ga和其他FSMA活性材料的理解和潜在应用产生广泛影响。此外，研究的教育和推广影响将远远超出研究生的培训。我们在研究中大量使用本科生，并在全球（中国、芬兰、乌克兰、西班牙）和美国（加州大学圣地亚哥分校、亚利桑那州立大学、爱荷华州立大学、马里兰大学）拥有广泛的合作伙伴；他们中的许多人将参与拟议工作的部分工作。最后，我们的团队荣幸地多次受邀在国际会议上发言，介绍我们在这一领域的工作成果（德累斯顿、新加坡、坎昆、卡迪夫和美国的许多地方）。这为更广泛地传播我们的成果提供了宝贵的机会。

TECHNICAL: Ferromagnetic shape memory alloys (FSMAs) have been extensively studied, particularly since the observation of magnetic-field-induced strains of 0.2% in Ni2MnGa crystals at -150C in 1996. More recently strains of 6% and 9.6% have been observed at room temperature in off-stoichiometry, martensitic compositions having tetragonal 5-M structure and orthorhombic 7-M structures, respectively. The mechanism for these large field-induced strains is believed to be twin boundary motion driven by magnetic energy differences and torques across the twin boundaries for certain applied field directions. While phenomenological theories exist that account for the general field dependence of the strain and magnetization, the microscopic mechanisms driving the twin boundary motion are not yet well understood. It is also not clear whether, and if so, how, the chemical order in stoichiometric compositions and chemical order in off-stoichiometric compositions affects the nucleation or kinetics of the partial dislocations, which control twin-boundary motion. This grant supports a multi-faceted experimental and modeling approach to address these issues. Neutron scattering is carried out at NIST (or Los Alamos) to determine the statistical site disorder in typical 5-M and 7-M compositions. These compositions always have excess Mn and deficiencies in Ga with Ni generally ranging from 48 to 51%. This determines where the excess Mn atoms prefer to reside, on Ni or Ga sites. An attempt will be made to identify species that accumulate at twin boundaries and that may account for the observed magnetic field or stress-induced creep observed in these materials. The effects of excess Mn occupation of Ni or Ga sites on saturation magnetization and magnetic anisotropy are studied with differential magnetometry. The Mn atoms in this intermetallic Heusler composition carry the majority of the magnetic moment. Thus, it is expected that whether Mn atoms occupy Ni or Ga sites, they will be closer to other Mn atoms and hence there could be a magnetic moment decrease. The reduced site symmetry in the partially disordered compositions may also lead to a reduction in the magnetic anisotropy, which is critical to field induced twin boundary motion. The application of a small acoustic signal to a Ni-Mn-Ga crystal, directed to induce a shear motion parallel to the twin planes has a significant enhancing effect on magnetic-field-induced twin boundary motion. These changes are investigated with changing stoichiometry. The experimental results are interpreted in terms of evolving microscopic models of partial-dislocation-mediated twin-boundary motion. NONTECHNICAL: This research is expected to have broad impact on the understanding and potential applications of Ni-Mn-Ga and other FSMA, active materials. Further, the education and outreach impact of the research will extend well beyond the training of graduate students. We make significant use of undergraduate students in our research and have extensive worldwide (China, Finland, Ukraine, Spain) and US (UCSD, ASU, Iowa State, UMd) collaborators; many of them will be involved in parts of the proposed work. Finally, our group has been honored with numerous invitations to speak at international conferences on the results of our work in this field (Dresden, Singapore, Cancun, Cardiff, and numerous locations in the US). This provides valuable opportunities for wider dissemination of our results.